<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" name="hello" value="Hello world" id="hello">
<script>
    var el=document.getElementById("hello");
    alert( el.value);
    el.value="你好"
</script>
</body>
</html>